1. Field of the Invention
This invention relates to the detection of alternating currents. More particularly, this invention relates to the detection of alternating currents that provide electrical power.
2. Description of the Background
Clamp-on current sensors are current sensors that can be placed around a current carrying wire while the wire is in a circuit. There is no need to remove the current carrying wire from the circuit in order to place a clamp-on sensor onto the wire. Clamp-on current sensors function by detecting the magnetic flux generated by the current carrying wire. In order to do this the clamp-on sensor must encircle the current carrying wire and provide a complete magnetic flux path around the current carrying wire. This is generally accomplished with sections of a magnetic material which clamp around the current carrying wire. Examples of such sensors appear in U.S. Pat. Nos. 2,709,800, 4,456,873, 4,513,246, 4,646,006, 4,794,329, 4,998,060, and 5,180,972, the teachings of which are hereby incorporated by reference.
The magnetic core material of a clamp-on sensor, in effect, amplifies the magnetic flux that is generated by the current carrying wire due to the magnetic permeability of the magnetic core material. That is, a magnetic flux, sometimes called a magnetic current, is induced in the magnetic core material in response to the magnetic flux of the current carrying wire. The changes in the total magnetic flux passing through the magnetic core of the clamp-on sensor is then detected by a magnetic flux transducer, amplified and/or processed to provide a suitable output voltage or current in a range corresponding to the measured current, all within the clamp-on sensor, and then the suitable output is made available for measurement external to the clamp-on sensor.
Conventional clamp-on transducers provide output voltages on the order of tens of millivolts. In order for clamp-on current sensors whose transducers have these low output voltages to be compatible with standard semiconductor voltage ranges of e.g., 0 to 5 volts DC, or -5 to 5 volts AC, their output must be amplified which requires active electronics in the clamp-on sensor requiring a source of power to the sensor. Moreover, available current sensors and, more particularly, clamp-on current sensors have a rather limited bandwidth. One reason for the limited bandwidth is because of the need for amplification of the low transducer output voltage and another reason is the type of transducers that are used.
Because clamp-on current sensors require mechanical opening and closing of the magnetic core sections forming the primary of their circuits they have a problem in reproducibility of their output signals. That is because the joints where the magnetic core sections of these clamp-on sensors are clamped together have a large magnetic reluctance associated with them (relative to the internal magnetic reluctance of the magnetic core sections) and the magnetic reluctance of these clamped regions is a sensitive function of the spacing and alignment of opposing surfaces of the magnetic core sections. Thus, each time a clamp-on sensor is clamped onto a current carrying wire, its output response to the current in the current carrying wire changes.
Modern developments in power transmission, especially power transmission inside of buildings has increased the efficient use of space. One result of this trend is that power cables are now lagged much closer to walls and much closer to one another than in the past. For example, power carrying cables may now be suspended from a wall with no more than a half an inch between the wall and the cable. In addition, power cables may be strung in buildings so that they are parallel to one another, running along a common wall, and separated by no more than half an inch. While these trends provide more efficient utilization of space in building designs, they conflict with the growing need to monitor the power provided to the building, by restricting the space available for a clamp-on current sensor.
Power monitoring is needed to protect sensitive electronic devices and to maintain efficient power consumption. Many buildings are now being built with a backup power source which can substitute for power provided from a power utility. Power monitoring allows the building to automatically switch from utility power to backup power when it is found that the utility power is unsatisfactory. In addition, many buildings are being retrofitted with power monitor and control systems.
Due to the close spacing of the permanently fixed power cables in modern buildings, it is difficult to fit a current sensor around them in order to monitor the current passing into and through the building. Many clamp-on sensors are of a type having handles which, when squeezed together, open two magnetic core sections so that the magnetic core sections may be placed around a lagged current carrying wire. However, these clamp-on sensors do not fit around the current carrying wire when there are parallel wires above and below the current carrying wire that is to be clamped. Moreover, conventional clamp-on sensors, even if they could be clamped around such cables, do not provide sufficient reliability, one to another and with respect to each time that they are clamped, and do not provide sufficiently large bandwidth response.
Large clamp-on current sensor bandwidth is very important. That is because one of the most important functions of power monitoring is to prevent high-frequency spikes in current occurring in the line power from reaching the electronic devices in a building. To that end it is necessary to have a current sensor that provides a high-frequency response. In this regard, high-frequency pulses on a power line tend to overload conventional clamp-on sensors by, for instance, overloading the active amplification circuits that amplify the relatively low millivolt transducer outputs of those devices. Consequently, such devices have a limited range of current that they can sense at high frequency, and also the active amplification circuits are easily damages by the spikes on power lines. Since the spikes on power lines are the most dangerous condition for building electronics, and the event that it is most desirable to protect against, conventional clamp-on sensors have only limited utility in the field of power monitoring.
It is also necessary to have reliability between various clamp-on current sensors' responses so that, for example, accurate measurements between the three different phases of power entering a building may be obtained. It is to overcoming thee problems that the present invention is directed.